I would personally say 2. My order is 2, Reach, 3, 4, ODST, CE, and I haven't played 5 yet.
-
I's say reach. I loved how it started as a mission to find out what happend to visegard relay outpost while suspecting that rebels destroyed it but it was actually the covenant. Another great thing from that campaign was that it feeled like there was hope from start to the long night of solace mission. After that the feeling of the campaign was that you're losing the war and doing everything to buy more time to evacuate the planet.